1. Field of the Invention
The present invention relates to a common rail fuel injection system. In particular, the present invention relates to a common rail fuel injection system of a pressure-increase type in that a pressure-increase mechanism increases the pressure of high-pressure fuel supplied from a common rail.
2. Description of the Related Art
A common rail fuel injection system that accumulates high-pressure fuel that is pressure-fed from a supply pump and injects the fuel into cylinders of an engine through fuel injection valves in predetermined timing depending on the operating state of the engine has been put into practical use. This type of fuel injection system is becoming mainstream in the field of diesel engines for a vehicle because it is capable of controlling injection pressure and injection timing independently of each other, but there is still room for improvement in terms of NOx reduction and combustion noise reduction because, for example, the initial injection quantity is large due to injection pressure waveforms being substantially rectangular.
Therefore, a common rail fuel injection system of a pressure-increase type that is capable of controlling injection pressure waveforms has been developed as disclosed in, for example, Unexamined Japanese Patent Publication No. 2002-364484 (hereinafter referred to as Patent Publication 1). This type of fuel injection system is configured such that the pressure of fuel supplied from a common rail is increased by a pressure-increase mechanism, so that injection pressure waveforms of fuel can be controlled by arbitrarily setting whether or not pressure is to be increased by the pressure-increase mechanism and the timing of operation of the pressure-increase mechanism. The pressure-increase mechanism increases fuel pressure by a pressure-increase piston, and the elimination of fuel pressure acting as back pressure on the pressure-increase piston operates the pressure-increase piston to pressurize fuel.
The pressure increase by the pressure-increase mechanism mentioned above is carried out in the case where the required injection pressure of fuel to be supplied to cylinders cannot be achieved only by common rail pressure. Specifically, in accordance with a map of FIG. 3, when the target injection pressure is increased with increase in the operated amount of an accelerator pedal (required load) and engine speed, the pressure-increase mechanism starts increasing fuel pressure based upon a pressure-increase flag set in a map of FIG. 4. For example, when a vehicle is running at low speed, a relatively low target injection pressure is set because engine load and engine speed are low, and therefore fuel injection is carried out with the pressure-increase mechanism being at a standstill. When the accelerator pedal is depressed in this state so as to accelerate the speed, the target injection pressure is increased in response to a rapid increase in required load, and the pressure-increase mechanism starts increasing fuel pressure so as to achieve the increased target injection pressure as shown in a time chart of FIG. 13.
The elimination of fuel pressure acting on the pressure-increase piston leads to consumption of pressurized fuel other than in fuel injection, and therefore the amount of pressurized fuel consumed considerably increases upon the start of pressure increase by the pressure-increase mechanism, and the quantity of fuel discharged from a supply pump is rapidly increased by necessity so as to maintain a predetermined common rail pressure. As a result, there is the problem that as shown in FIG. 13, when the amount of pressurized fuel consumed is abruptly changed in response to activation and deactivation of the pressure-increase mechanism, the quantity of fuel discharged from the supply pump (driving load) is abruptly changed, causing torque shock and rotational fluctuation to occur in an engine that drives the supply pump and therefore deteriorating drivability.
Also, the engine has a property of changing the state of combustion and thereby changing combustion noise depending on fuel injection pressure, and therefore when the target injection pressure is abruptly changed in response to depression of the accelerator pedal or the like, combustion noise is also abruptly changed, causing a driver to feel something is wrong.